Spirometry: Understanding the curves

Spirometry is used to assess how air moved in and out of the lungs. It measures how much air is exhaled (volume) and also how fast it gets out of the lungs (flow = volume/time). The following illustration describes what does each curve measure and how it changes as the patient is puffing really hard into the machine. Use this as a review after your read about spirometry from a proper textbook! ☝️

The volume-time graph: Assesses how fast air is coming out. You can ser that almost everything has been exhaled within the first second (FEV1). After 6 seconds, the amount of air exhaled is known as the forced vital capacity (FVC) (Lets say it can’t be “total” because there is still some air inside the dead space of the airway that won’t come out). Using FEV1/FVC you can get the Tiffeneau ratio which will help you find out if there is an obstructive problem (Which means air is not coming out from the lungs properly).

The volume-time graph – Click to enlarge and download.

The flow (air speed)-volume loop: In this curve time is not represented, but is included in the flow (v/t). You can see the moment in which the flow is fastest (peak espiratory flow, PEF) and that will usually correspond tothe very beginning of the patient’s puff, when less than 25% of the volume has been exhaled from the lungs. It’s also used together with the other graph to assess the validity of the test by looking at the proper shapes of the curves. Finally, some diseas might affect the flow mainly when 25-75% of the volume is being exhaled. This loops helps visualizing such data.

The flow-volume loop – Click to enlarge and download.

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